Screening can have 2 meanings. One is for the general process of overall making sure that your organization is attracting and selecting the right applicants for the right roles. This is done through your various recruitment steps, and is generally called 'screening and selection'.
Screening a form is simply the process of making sure you've looked at an applicant's form, 'screened' it (i.e. read it carefully) and decided if the applicant is appropriate to progress through your volunteer recruitment funnel.